This invention relates to a method and apparatus for casting a thin polymer film and more particularly to a method of casting a polymeric membrane on the inside surface of porous tubes to provide a permeate separation system having excellent selectivity capabilities.
Several methods for casting a thin dense polymer film are known. For example, a substrate can be dipped in a polymer solution, a polymer solution can be spray cast upon a surface or substrate; an aliquot of a polymer solution can be placed upon a support and the support rotated at high speeds to fling off excess solution; a polymer solution can be dropped onto a high speed rotating flat substrate; a draw down bar can be used; a thin film can be extruded; or a continuous substrate can be exposed to the solution and allowed to drain off. However, none of these methods can be applied effectively to the inside surface of a porous tube.
Current methods for casting polymeric membranes on the inside surface of tubes include simply pouring the polymer solution into the tube and then pouring it back out again, repeating the process as needed. Another method is a pressure system that can be used to force the viscous solution up into the tube and then force the solution back out. These techniques however, do not provide a uniformly thin polymer film thickness and therefore result in an uneven polymer morphology, and accordingly polymeric membranes produced by these current techniques have less desirable permeate selectivity capabilities.
Also, current methods of casting membranes on the inner surfaces of tubes, such as that described in U.S. Pat. No. 4,410,568 of Iwama et al., require that the membrane be physically detached from the tube in which it was cast so that it can be slipped into a perforated stainless steel tube. A disadvantage of this technique is that it is limited to membranes that are physically capable of enduring the detachment from a glass tube and subsequent transfer into a perforated stainless steel tube. Because many useful membranes have a thickness in the range of 1 to 100 microns and therefore cannot be physically manipulated in the manner disclosed in Iwama et al., it is desirable to permanently attach, or adhere, membranes having the desired uniformity and separation capability directly onto the inside surface of a porous tubular support.
It is an object of this invention to provide a method for casting a polymer membrane of uniform thickness on the inside surface of a porous tube.
It is another object of this invention to provide a method for casting a continuous homogeneous polymer membrane on the inside surface of a porous tube to create a semipermeable membrane system having enhanced selectivity capabilities.
It is still a further object of this invention to provide a method for producing an energy efficient permeate separation having excellent selectivity capabilities.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.